MCrAlY type coatings are commonly used as overlay coatings and bondcoats for the protection of components against high-temperature oxidation and corrosion. During exposure to high temperatures, an oxide scale forms on the MCrAlY surface, becoming part of the original system. This thermally grown oxide is largely comprised of alumina and often represents the starting point for failure. Correspondingly, the growth rate and adhesion of the oxide scale and the aluminum depletion in the bondcoat are among the determining factors for the useful lifetime of am MCrAlY bond coated article.
At long term exposures, the alumina scales are prone to spallation. The scale spallation is a common event under temperature cycling conditions due to the thermal expansion mismatch between the oxide and the metallic substrate, and failure is generally related to scale thickness, growth rate and adherence. The two latter parameters are crucially affected by the addition of Yttrium, as well as other reactive elements such as Lanthanum, Zirconium, and Hafnium. These reactive elements will commonly constitute some percentage, typically less than 0.5 wt. % to the base MCrAl-composition. These reactive elements have been found to greatly improve alumina scale adherence and significantly increase the critical scale thickness for spallation. However, this beneficial effect is highly sensitive to the Y-concentration. In some cases, optimum oxidation and improved adherence is obtained by low concentrations of Y in the coating (around 0.1 wt. %), whereas a few tenths of a weight percent more (over 0.5 wt. %) can lead to accelerated oxidation and detrimental oxide morphology. This implies that relatively low amounts of Yttrium are desired in the coating. Issues have arisen as a result of this Yttrium content sensitivity however, because Yttrium can be tied up by Y—Al oxides resulting from typical fabrication processes, and the effective Yttrium concentration of the MCrAlY coating may significantly deviate from that of the original MCrAlY source material.
Depletion of the Yttrium reservoir through the formation of excessive Y—Al oxides will therefore promote TGO spallation in service. Such depletion results in deviation from the optimum Yttrium content designed to obtain a compromise between the beneficial effect on oxide adherence without extensively enhancing the growth rate of the oxide. Defining and maintaining this optimum yttrium content it is absolutely necessary to take into account the actual Yttrium reservoir in the MCrAlY coating. See e.g., Toscano et al., “Parameters affecting TGO growth and adherence on MCrAlY-bond coats for TBC's,” Surface & Coatings Technology 201 (2006), among others. It would be advantageous to provide an MCrAlY bond coat wherein the formation of Y—Al oxides is suppressed, and the Yttrium reservoir of the final heat treated composition could be largely defined by the Yttrium concentration of the original MCrAlY source material. Such an MCrAlY bond coat would mitigate variations in the Y-distribution and the reservoir of metallic Y in MCrAlY coatings and avoid the significant variations in the alumina scale growth rate and adherence which accrue from nominally equivalent MCrAlY bond coat compositions.
Additionally, a common feature of the oxide morphology in MCrAlY bond coats is the formation of Y2O3 compounds near or at the metal/oxide interface, known as pegs. The coefficient of diffusion of oxygen in Y2O3 is about 2.2×10−11 cm2/s, and is considerably higher than the surrounding Al2O3, where the value is generally around 1×10−17 cm2/s (at 700° C.). As a result, the presence of Y2O3 in the alumina scale is associated with locally accelerated oxidation along these Y-compounds, which leads to internal oxidations intruding in the coating alloy. In real oxide scales, this short-circuit path diffusion significantly impacts the kinetics of internal MCrAlY bulk oxidation. It would be additionally advantageous to provide an MCrAlY bond coat wherein an upper surface layer of Y-Al2O3 with an absence of Y2O3 could be formed, in order to mitigate internal MCrAlY bulk oxidations enabled by short-circuit path diffusion through Y2O3 in the upper layer. Such an absence of Y2O3 and other Y—Al oxides in the MCrAlY upper layer would further preserve the reservoir of metallic Y in MCrAlY coatings and promote MCrAlY effective lifetimes.
Provided here is an MCrAlY bond coat comprised of an MCrAlY layer in contact with a Y—Al2O3 layer. The MCrAlY layer is comprised of a γ-M solid solution, a β-MAl intermetallic phase, and Y-type intermetallics. The Y—Al2O3 layer is comprised of Yttrium atoms coordinated with oxygen atoms comprising the Al2O3 lattice. The MCrAlY layer and the Y—Al2O3 layer have a substantial absence of Y—Al oxides. The absence of selected Y—Al oxides within the MCrAlY layer provides advantage in the maintainability of the Yttrium reservoir within the MCrAlY bulk, and the absence of selected Y—Al oxides within the Y—Al2O3 layer mitigates oxygen diffusion and undesired modification of the originally intended Yttrium distribution in the underlying MCrAlY bulk.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.